In general, physical methods, especially cathode sputtering, have been used to date for the production of thin copper layers. However, this method has the disadvantage that, particularly in the production of copper starting layers for the interconnect system of highly integrated circuits—with increasing reduction of the geometrical dimensions—uniform closed surface layers are no longer obtained.
Chemical methods, for example, variants of chemical vapor deposition (CVD), are suitable as alternative methods for producing such layers. Here, source substances or precursors which contain the desired metal (e.g., copper) in the form of a chemical compound are fed in the gaseous state to a vacuum chamber which is in the form of a hot- or cold-wall reactor and in which the layer deposition is subsequently effected. For this purpose, the precursors are converted into the gas phase prior to deposition. Accordingly, a layer formation reaction takes place on the surface of the heated wafer substrate. This may consist in targeted thermally controlled decomposition of the precursor; often, reducing or oxidizing agents are also necessary for the layer deposition. However, the CVD methods have the disadvantage that the layer growth is not uniform here and closed surface layers form only from a thickness of a few 10 nm.
By using atomic layer deposition (ALD), these disadvantages can be avoided. This is a cyclic method in which in general two reactants are fed to the reaction chamber in pulses. These pulses are separated from one another by inert purging and/or evacuation steps so that the two reactants never meet one another in the gas phase and exclusively surface reactions of the second reactant with adsorbates of the first reactant lead to layer formation. The first reactant is initially chemisorbed on the substrate surface so that the substrate is substantially covered with a monolayer of the precursor. Further monolayers which form by physisorption are removed during the purging or evacuation pulses. It is therefore necessary for the precursor to be able to undergo chemisorption on the substrate to be coated. By means of the ALD method, it is therefore possible to control the desired layer thickness very accurately via the number of ALD cycles.
In order to produce copper layers by means of ALD, in general two approaches can be chosen for the deposition. Either elemental copper can be produced directly during the individual ALD cycles; alternatively first a copper species can be produced (e.g., copper oxide—CuOx), which is then reduced to copper. The first variant for the production of elemental copper is, however, generally difficult.
U.S. Pat. No. 6,869,876 B2 describes a method of the generic type in which first a copper halide layer is produced on the substrate and is then reduced by means of a reducing agent to give a copper layer. Copper(I) and copper(II) complexes are used here as a precursor. In particular, complexes of the type LCu(X∩X) are mentioned here as copper(I) complexes. The bidentate ligand X∩X here represents β-diketonates, and hexafluoroacetylacetonate (hfac) is mentioned explicitly. The ligand L is a stabilizing ligand, for example, an olefin, such as trimethylvinylsilane (tmvs). The reduction can be effected, for example, with diethylsilane.
U.S. Pat. No. 6,482,740 B2 describes an ALD method of the generic type in which a copper oxide layer is first obtained. Here, copper(I) and copper(II) compounds are used as a precursor. For example, (PEt3)Cu(hfac) is mentioned as a copper(I) compound. For producing the oxide layer, in each case an oxidation pulse with water, H2O2, O2, O3 or similar oxidizing agents is carried out during an ALD cycle. For reducing the copper oxide layer, reducing agents such as ammonia, hydroxylamine, hydrazine, alcohols (e.g., methanol), aldehydes (e.g., butyraldehyde), carboxylic acids (e.g., formic acid or acetic acid) and hydrogen are used. The reduction is effected at temperatures of from 310 to 450° C.
The above methods have the disadvantage that fluorine-containing precursors are used. Fluorine can accumulate at the interface with the substrate material and reduce the adhesion of the copper layer to the substrate there.
WO 2004/036624 A2 discloses an ALD method in which a precursor which is not fluorine-containing is used. Homoleptic copper complexes, for example, copper(II) β-diketonates and copper(I) tert-butoxide, are proposed as the precursor here. Ozone, oxygen, water or mixtures thereof are used for the oxidation pulse; the reduction is effected by means of a hydrogen-containing gas.